2017MNRAS.471.2321D

We report the discovery of an extremely large (R_b ∼2.77 arcsec ≃ 4.2 kpc) core in the brightest cluster galaxy, IC 1101, of the rich galaxy cluster Abell 2029. Luminous core-Sersic galaxies contain depleted cores - with sizes (R_b) typically 20-500 pc - that are thought to be formed by coalescing black hole binaries. We fit a (double nucleus) + (spheroid) + (intermediate-scale component) + (stellar halo) model to the Hubble Space Telescope surface brightness profile of IC 1101, finding the largest core size measured in any galaxy to date. This core is an order of magnitude larger than those typically measured for core-Sersic galaxies. We find that the spheroid's V-band absolute magnitude (M_V) of -23.8 mag (∼25 per cent of the total galaxy light, i.e. including the stellar halo) is faint for the large R_b, such that the observed core is 1.02 dex ≃ 3.4σ_s (rms scatter) larger than that estimated from the R_b-M_V relation. The suspected scouring process has produced a large stellar mass deficit (M_def) ∼4.9 x 10¹¹ M_☉, i.e. a luminosity deficit ≃28 per cent of the spheroid's luminosity prior to the depletion. Using IC 1101's black hole mass (M_BH) estimated from the M_BH-σ, M_BH-L and M_BH-M_* relations, we measure an excessive and unrealistically high number of 'dry' major mergers for IC 1101 (i.e. N 76) as traced by the large M_def/M_BH ratios of 38-101. The large core, high mass deficit and oversized M_def/M_BH ratio of IC 1101 suggest that the depleted core was scoured by overmassive SMBH binaries with a final coalesced mass M_BH ∼ (4-10) x 10¹⁰ M_☉, i.e. ∼ (1.7-3.2) x σ_s larger than the black hole masses estimated using the spheroid's σ, L and M_*. The large core might be partly due to oscillatory core passages by a gravitational radiation-recoiled black hole.